The present invention relates to a process for the preparation of secondary and tertiary amines, and more particularly, but not by way of limitation, to a process for the preparation of dimethylcyclohexylamine from cyclohexanone and dimethylamine. 2. Description of the Prior Art
Secondary and tertiary amines have a wide variety of industrial uses, i.e., as reactive intermediates, acid neutralizers in syntheses, polymerization catalysts in the preparation of polyurethanes, in bactericides, in pharmaceuticals, in insecticides, and as corrosion inhibitors. Accordingly, a number of processes have been proposed for the preparation of secondary and tertiary amines from less substituted amines, and these processes employ various catalysts under a variety of conditions.
One group of such processes has proposed employing catalysts from the transition metals of group 8 of the periodic table, and in particular, nickel in the form of Raney nickel has been employed as a hydrogenation catalyst. Raney nickel and other catalysts from this group, however, tend to lead to the production of by-products. For instance, in the preparation of dimethylcyclohexylamine from cyclohexanone and dimethylamine, it has been found that the use of Raney nickel results in yields of from 10 to 20 percent cyclohexanol through the reduction of unreacted cyclohexanone. Because of the closeness of the boiling points of the dimethylcyclohexylamine and cyclohexanol products, separation by distillation is made impracticable. In addition, it has been found that Raney nickel tends to deactivate on standing in the presence of the product mixture in the dimethylcyclohexylamine process. Raney nickel is also highly pyrophoric.
A second group of processes has suggested employing oxides possessing dehydrating properties, for example, aluminum oxide or silicon dioxide, usually at high temperatures and high pressures. As a result of the conditions under which this group of processes is typically conducted, energy and equipment demands are relatively high. In addition, the high temperatures employed tend to lead to a greater variety of products and undesired reactions.
A third group of processes known to the art has suggested some sort of copper-based catalyst For example, U.S. Pat. No. 4,234,727 to Toussaint et al. discloses the use of a supported copper catalyst for the gas phase production, at from 100.degree. C. to 250.degree. C. and atmospheric pressure, of dimethylcyclohexylamine from dimethylamine and cyclohexanone.
U.S. Pat. No. 3,520,933 to Adam et al. discloses the use of a supported copper-containing catalyst which further contains 0.1 percent to 15 percent of a pyroacid or polyacid in the liquid phase reaction of an alcohol or carbonyl compound with ammonia, a primary or a secondary amine. The total pressure is disclosed as ranging from 20 to 400 atmospheres, and the temperature from 80.degree. C. to 230.degree. C.
U.S. Pat. No. 4,505,860 to Klein et al. discloses the use of Raney copper to catalyze the reductive amination of the product of the reaction of certain cyclic keto-butyraldehydes and ammonia, thereby producing a primary diamine. The diamine is then reacted with phosgene to produce a cyclic diisocyanate cross-linking agent.
U.S. Pat. No. 4,138,437 to Strauss et al. discloses a process for the formation of tertiary amines wherein liquid phase alcohol or aldehyde reagents having 7 to 23 carbon atoms are reacted with a gaseous mixture of hydrogen and a lower primary or secondary alkylamine having at least one methyl group in the presence of a copper-chromium oxide catalyst. The addition of metal oxides of the first and second groups of the periodic system such as potassium, magnesium or barium is also suggested. The temperature is disclosed as being within the range of 160.degree. C. to 230.degree. C., with the partial pressure of hydrogen being within the range of 1-5 atmospheres. The catalyst is disclosed as being suitably supported or unsupported by a carrier.
As shown by these disclosures, it has been generally known within the third group of processes described above to use a supported copper-containing catalyst in combination with a condensation promoter such as an acid or in combination with an activity-boosting metal oxide in the synthesis of secondary and tertiary amines from carbonyl compounds. One problem encountered with such supported catalysts in batch reactions is that such catalysts frequently will be reduced in particle size during the reaction to such an extent that gravitational settling of the catalyst at the end of the reaction is inhibited. Separation of the catalyst from the product mixture is thereby made more difficult, time-consuming, and expensive. A problem has also been noted with the addition of alkaline reagents such as the oxides just mentioned, in that such additives can cause the catalyst composition to agglomerate so that the activity of the catalyst composition rapidly declines in continuous operation.
As shown by the above-mentioned disclosures, there is a need for a catalytic process which does not result in the significant production of unwanted by-products, which can be performed at lower temperatures to lower energy consumption and reduce the production of by-products, and in which catalytic activity is not significantly diminished on shutting down a reactor. Further, there is a need for such a process that permits the efficient and effective separation of the products and residual reactants from each other and from the catalyst.